An electrocardiograph is a device whose electrodes are set on a biological body to measure an electric potential difference between the electrodes. A signal measured by an electrocardiograph is referred to as an ECG (Electrocardiogram) signal and is widely used in the medical field. An ECG signal has waveforms referred to as a P-wave, an R-wave, a QRS complex wave, and a T-wave, for example. Since these waveforms are used for a synchronization signal of a medical imaging apparatus capable of ECG synchronization imaging in addition to diagnosis of various types of cardiac disease, automatic detection of such waveforms is important in terms of industrial applications.
For example, in cardiac image diagnosis with the use of an MRI apparatus, imaging is performed at each timing synchronized with systole or diastole by using a synchronization signal (which is also referred to as a trigger signal) detected from an ECG signal. Such imaging is called ECG synchronization imaging.
In an ECG signal acquired from an examinee set inside an MRI apparatus, noise is mixed due to effects of magnetic fields generated by the MRI apparatus and this degrades signal-to-noise ratio. In particular, strong noise is mixed during imaging, and even an R-wave which is the most distinct waveform in an ECG signal becomes difficult to be stably detected in some cases. In order to robustly detect an R-wave with respect to noise, it is effective to enhance cardiac action potential in an acquired ECG signal and suppress noise while keeping the cardiac action potential undisturbed.
Noise which degrades performance of detecting an R-wave is generated due to RF (Radio Frequency) pulses and switching of gradient magnetic fields in association with imaging. Magnetic flux density around an electrocardiograph and its electrodes temporally changes, which generates induced electromotive force, and the induced electromotive force is mixed into an ECG signal as noise. Thus, there is similarity between characteristics of noise and characteristics of gradient magnetic fields and/or RF pulses. Accordingly, what type of noise is mixed into an ECG signal can be estimated to some extent from a gradient-magnetic-field control signal. So far, a method of estimating noise mixed into an ECG signal from a gradient-magnetic-field control signal by using an adaptive filter and removing the noise has been proposed.
However, in a conventional adaptive filter, amount of eliminated noise is not sufficient in some cases. Additionally, in a conventional adaptive filter, noise is excessively removed and cardiac action potential is negatively affected to a great extent in some cases.